The present invention relates to devices for countering vibration in structural members, such as those of an aircraft fuselage; and particularly to such devices which can be dynamically tuned to adapt performance to changes in vibration frequency.
Aircraft engines can induce significant vibration into the fuselage. In propeller powered planes, the propeller blades produce air pressure patterns which strike external surfaces thereby causing a time periodic vibration of the structure, at about 100 Hz for example, which vibration is transferred to other structural members of the airframe. Jet engines also produce vibration in supporting members. If left unchecked, the induced vibrations create objectionable noise in the aircraft cabin, and may result in serious fatigue of the airframe.
As a consequence, vibration absorbers are attached to structural members throughout the aircraft. These devices typically are a simple spring-mass system in which a mass is attached to the airframe by a resilient member that acts as a spring. Elastomeric pads and metal cantilevers have been employed as the spring. The spring-mass system is fixedly tuned to resonate at the frequency of common vibration in the structural member of the airframe to which the absorber is attached and thus optimally absorbs the vibration energy at that tuned frequency. The absorber has a large mechanical impedance at resonance which is due mostly to a high quality factor Q. Absorption (mechanical impedance) at other frequencies diminishes as a function of the deviation from the resonant frequency.
U.S. Pat. No. 3,490,556 discloses passive vibration absorbers 10 attached to pylons 12 to which the jet engine connects at the aft portion of the fuselage 14 on the Douglas Aircraft DC9-V series airplane as shown in FIG. 1. Four masses 16 are supported from each jet engine support yoke 18 by a separate cantilevered beam 19 which acts as a spring. These absorbers 10 are tuned to vibrational frequencies produced by the two engine rotors N1 and N2 which occur at the nominal cruise power setting of the engine.
One drawback of fixedly tuned absorbers is that the airframe vibration frequencies vary with engine speed, especially in the case of jet engines. Although the absorber may be tuned to the vibration frequency which occurs at the nominal cruising speed of the aircraft, less than optimal vibration absorption occurs at other speeds. In addition, the tuning of elastomeric type absorbers changes with the age of the elastomeric material, and tuning of both elastomeric and metal spring type absorbers change with temperature.
Therefore, it is desirable to provide an absorption system which dynamically adapts to variation of the vibration frequency.